Mechanically Speaking: Volkswagen emission scandal raises a stink

"As everyone probably knows by now Volkswagen is in hot water."

As everyone probably knows by now Volkswagen is in hot water.

Well not only Volkswagen but the Volkswagen Group. In the business we call them VAG (Volkswagen Aktiengesellschaft). The Volkswagen Group sells passenger cars under the Audi, Bentley, Bugatti, Lamborghini, Porsche, SEAT, Škoda and Volkswagen marques.

When driven on the highway their small diesel vehicles (TDI versions of Jettas, Golfs, and A3s in our neck of the woods)  are not meeting the emission standards for which they were certified. In particular they were not meeting the NOx (nitrous oxides) emission standards laid out by the EPA (The United States Environmental Protection Agency). In Canada we basically aligned our emission standards with the United States.

These diesel vehicles in the FTP (Federal Test Procedure) must produce less than 0.04 grams of nitrous oxides per kilometre. The FTP comprises a drive cycle that incorporates various types of driving (cold start, some idling, some cruising and some acceleration and deceleration). The test is done in a closed controlled environment where the vehicle is driven on a set of rollers and all the emissions are collected from the vehicle in bags. The regulated  pollutants are separated and measured.

It seems as if Volkswagen wrote software in their vehicle computer systems that recognized when the vehicle was being run on rollers. My first thought was they used the inputs from the vehicle wheel speed sensors and the stability control system. It is likely that the rolling is only done on two driving wheels.

When the vehicle is being tested then only two wheels would be rolling and the other two would be stationary. In this situation the software would use the full emissions capability of the vehicle. Rolling down the road all four wheel speed sensors would normally be reporting and in this condition the vehicle would reduce the ability of the emission system to eliminate nitrous oxides.

Why? If the emission system is capable why turn it off? Volkswagen chose both EGR (Exhaust Gas Recirculation) and a Nitrous Oxide Absorber to meet the NOx emission standards. Implementing these two systems reduces power on one hand and fuel economy on the other. As discussed in a previous article EGR reduces combustion chamber temperature (the explosion temperature in the cylinder). Lower temperatures stop the formation of NOx. EGR also dilutes the air entering the cylinder and therefore lowers efficiency (fuel mileage). In the trade we also see that EGR systems gradually cause deposit formation in the intake path. These deposits disturb airflow into the engine and hurt performance.

After the EGR any leftover NOx is treated in the NOx Absorber. The absorber is a honeycomb device similar to a catalytic converter.

The NOx is collected on the metallic surfaces of the honeycomb. When the surface starts to be saturated with NOx excess fuel is squirted into the exhaust system which starts a catalytic reaction in the absorber which converts the NOx to Nitrogen and water.

That excess fuel is not making power for you and ultimately reduces fuel economy.

So lowering the EGR rate and turning off the excess fuel to the absorber will increase fuel economy and power, reduce intake deposits but raise emissions. Science always has its compromises.

I guess the powers that be at Volkswagen felt the upside of increased emissions was too hard to resist. Those TDI owners were certainly enjoying their vehicle’s high power and high fuel economy.

I wonder how all this is going to play out.

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